266 ANNALS OF THE CARNEGIE MUSEUM. 
obsolete; when present, consisting of few fine concentric ridges, to 
which may be added, toward the disk, a small number of slightly 
heavier bars, with a posterior angle upon the posterior ridge of the 
shell; these bars run about parallel to the growth lines. Sometimes 
the bars are slightly sinuate in front of the posterior angle, but they 
never are distinctly of the double-looped type. Nacre from white 
through all shades of pink and red to deep purple and violet, with the 
dark shades prevailing. 
Soft parts practically identical with those of Plethobasus and Pleuro- 
bema, with only the outer gills marsupial. Mantle-connection between 
anal and supra-anal short, or somewhat longer. Inner lamina of 
inner gills free. Color of the soft parts whitish, often greatly suffused 
(chiefly the gills and mantle) with black. Eggs, where known, whitish, 
placente rather distinct. Glochidia small, subovate, without hooks 
(see Plate XIX, fig. 1). 
It may be mentioned as an additional character, that in all these 
forms the anal has rather distinct papille. 
Type: E. crassidens (Lamarck.)" 
We may regard Eiliptio as a special branch of Pleurobema, distin- 
guished from the latter only by the characters of the shell. It prob- 
ably is not descended directly from a Quadrula- or Fusconaja-like type 
with four marsupial gills, but it has gone through the intermediate 
Pleurobema-stage first. 
The species of Elliptio easily fall into several groups. 
In E. crassidens and beadleianus, the typical shape of the shell is 
not so distinctly developed: it is not greatly elongated, and approaches 
yet somewhat the subquadrate or subtrapezoidal type of certain 
Fusconaja- and Pleurobema-species. The beak sculpture in these 
forms is rather obscure. 
Next to this stands the group of E. complanatus (including productus 
and jayensis), where the typical characters of the genus are fully de- 
veloped. 
A third type is furnished by E. gibbosus, and a fourth by E. popet. 
Elliptio crassidens (Lamarck). 
Numerous specimens both males and females have been examined, 
all from the Ohio and Allegheny Rivers in western Pennsylvania. 
1 Unio (Elliptio) nigra Rafinesque, 1820, is Rafinesque’s type (first species), 
and this is undoubtedly a synonym of U. crassidens Lamarck, 1819. The large, 
heavy shell of the Ohio with red nacre cannot be anything else. 
